Thin film photovoltaic (PV) modules (also referred to as “solar panels”) are gaining wide acceptance and interest in the industry, particularly modules based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components. Solar energy systems using CdTe photovoltaic (PV) modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner.
Certain factors greatly affect the efficiency of CdTe PV modules in terms of cost and power generation capacity. For example, CdTe is relatively expensive and, thus, efficient utilization (i.e., minimal waste) of the material is a primary cost factor. In addition, the energy conversion efficiency of the module is a factor of certain characteristics of the deposited CdTe film layer. Non-uniformity or defects in the film layer can significantly decrease the output of the module, thereby adding to the cost per unit of power. Also, the ability to process relatively large substrates on an economically sensible commercial scale is a crucial consideration.
CSS (Close Space Sublimation) is a known commercial vapor deposition process for production of CdTe modules. Reference is made, for example, to U.S. Pat. Nos. 6,444,043 and 6,423,565. Within the vapor deposition chamber in a CSS system, the substrate is brought to an opposed position at a relatively small distance (i.e., about 2-3 mm) opposite to a CdTe source. The CdTe material sublimes and deposits onto the surface of the substrate. In the CSS system of U.S. Pat. No. 6,444,043 cited above, the CdTe material is in granular form and is held in a heated receptacle within the vapor deposition chamber. The sublimated material moves through holes in a cover placed over the receptacle and deposits onto the stationary glass surface, which is held at the smallest possible distance (1-2 mm) above the cover frame. The cover is heated to a temperature greater than the receptacle.
While there are advantages to the CSS process, the related system is inherently a batch process wherein the glass substrate is indexed into a vapor deposition chamber, held in the chamber for a finite period of time in which the film layer is formed, and subsequently indexed out of the chamber. The system is more suited for batch processing of relatively small surface area substrates. The process must be periodically interrupted in order to replenish the CdTe source, which is detrimental to a large scale production process. In addition, the deposition process cannot readily be stopped and restarted in a controlled manner, resulting in significant non-utilization (i.e., waste) of the CdTe material during the indexing of the substrates into and out of the chamber, and during any steps needed to position the substrate within the chamber.
Accordingly, there exists an ongoing need in the industry for an improved vapor deposition apparatus and process for the economic large scale production of PV modules that reduces film defects and waste of the source material.